As experiment I found some really nasty rusty coins on the street. They were green. After looking on a website I found vinegar could eliminate the rust. It worked pretty well.

I know this is a redox reaction. Can you tell which ones? I know these are pesos of various denominations from Dominican Republic from the past 10 or 20 years. So that might help identify the metal.

The vinegar is household apple cidwe vinegar from the supermarket. Low concentration.

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    $\begingroup$ The coins are most likely copper. The blueish-green rust is the patina of copper which is a mixture of various copper compounds like copper sulfate, copper carbonate, etc. The acetic acid probably dissolves these salts. $\endgroup$
    – AvZ
    Feb 2 '16 at 18:17
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    $\begingroup$ BTW, this is not a redox reaction at all. $\endgroup$ Feb 2 '16 at 20:31
  • $\begingroup$ Vinegar and salt works better than vinegar alone. Not sure why but the question has been asked here before. $\endgroup$
    – matt_black
    Feb 2 '16 at 20:57
  • $\begingroup$ Adding salt introduces chloride ions, which form soluble salts or complexes with most metals. This mixture will not only remove patina, it will further corrode the coin's metal. Please DO NOT use it on any coins that might be valuable or collectible. $\endgroup$
    – jeffB
    Mar 21 '19 at 16:22

As previous comments said, this isn't a redox reaction.

The patina or corrosion is made of metal salts -- for copper, it's copper oxide, sulfide, carbonate, and perhaps other things. Vinegar readily converts all these to soluble copper acetate.

Most coins are made of metal alloys, and those alloys sometimes contain metals that are attacked by acid. Nickel and zinc are common components. Soaking in vinegar will dissolve these metals (they'll displace hydrogen, again forming soluble metal acetates). This damages the coin's surface, making it porous, and eventually reducing the coin's weight. (It also breaks collectors' hearts.)

Adding salt to the mix forms an equilibrium with hydrochloric acid, which attacks metals even more aggressively. Please don't do this unless you don't mind ruining your coins.


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